VLSI devices are quite common nowadays in electronic and computer systems for enhancing their functionality while reducing their dimension. However, as more and more systems operate on different voltage levels, a device, or a group of devices such as a modem chip set, must be able to detect the supplied voltage level and adapt itself accordingly. When the device is plugged in and the system is powered up, the device is typically reset by the system so that the device can configure its I/O's consistent with the voltage level of the power supply. Whether the power supply is at 3 volt or 5 volt, the device must detect the power supply before any program execution takes place. Once the device is properly configured, no other reconfiguration is necessary as long as the power is still on.
Typically, upon power-on, a system reset is also generated to reset the system and the plugged-in devices. This process is commonly known as a "cold start". A cold start is also followed by a brief delay of about 80-100 mS for stabilization and supply detection. There is, however, another form of reset, "warm reset", with the power still on, which can be invoked by the user or other controlling parts of the system. A warm reset does not and should not require the device to re-detect the voltage level of the power supply because 1) the power is still on and the existing configuration is still valid, and 2) reconfiguration would unnecessarily slow the system down.
Therefore, it would be desirable to be able to distinguish between a system reset caused by the power-on process ("cold start") and a system reset unrelated with power-on ("warm reset").
Also, it would be desirable to be able to ignore any subsequent system resets, "warm resets", once a cold start is completed.
Further, it would be desirable to have a supply sensing circuit which is robust against any transients from the power supply such that a warm reset would not be mistaken as a cold start.